My ID/library card/cash/ibuprofen/keys don't fit very comfortably in pockets. If I really wanted to travel light, I think a belt pack, handlebar pack or under seat pack would be the way to go for a chain lock.

My U-lock came with a fitting so it could just hook on the frame... but it's for a steel frame bike, not an aluminum fatty like mine. If your bike has a lighter frame, a U-lock might be easier to manage than the chain.